1. Field of the Invention
The present invention relates to a seal ring, mounted in a ring groove provided in a shaft of a device such as an automatic transmission (hereinafter, simply referred to as AT), for maintaining the oil pressure required for an operation of that device.
2. Description of the Related Art
Seal rings used in a device such as an AT are respectively mounted in a pair of ring grooves, provided on an outer circumferential surface (periphery) of a shaft of the device, to be distanced away from each other. The outer circumferential surface of the seal ring is in contact with an inner circumferential surface of a housing for accommodating a clutch plate or a brake plate slidable on that surface. Each of the seal rings receives operating (hydraulic) oil, supplied from an oil channel between the ring grooves, at one side face, i.e., a pressure-receiving side face, and the inner circumferential face thereof while sealing the side face of the ring groove and the inner circumferential surface of the housing with the opposite side face, i.e., the contact side face, and the outer circumferential surface. Sliding movement of the side face of the seal ring, with respect to the wall face of the ring groove, maintains the operating oil at an appropriate pressure between the seal rings.
Under this condition, the seal ring is required to show small frictional loss and to be able to maintain good sealing properties for a long time. When the seal ring is thus mounted, any frictional force between the side face of the seal ring and the wall face of the ring groove of the shaft has to be small. However, in the case of a conventional seal ring having a rectangular cross section, the contact area between the side face of the seal ring and the wall face of the ring groove is large. Consequently the frictional loss is large.
In recent years, improvements in fuel consumption by further reducing the weight of the AT and reduction of friction have been demanded from the viewpoints of improving vehicle performance and increasingly tighter environmental standards. Thus, for the seal ring, an improvement in its characteristics has been demanded so as to achieve both reduction in friction between the seal ring and the wall face of the ring groove and good sealing properties irrespective of the processing accuracy of the ring groove.
As an example of a typical conventional seal ring, Japanese Utility-Model Laid-Open Publication No. Hei 6-18764 discloses a seal ring, as shown in FIGS. 4 and 5. A seal ring 5′, mounted in a ring groove 4 of a shaft 1, has an annular groove 22 on its side face that extends in a circumferential direction, and a plurality of grooves 21 that are distanced away from each other in the circumferential direction so as to be directed in a radial direction. Through the radially directed grooves 21, the annular groove 22 is opened to the inner circumferential face 23 of the seal ring 5′. The side face of the seal ring 5′ is opposed to and in contact with the side face of the ring groove 4 through the radially directed grooves 21 and the annular groove 22. The reference numeral 2 denotes a housing, and 8 denotes a flow of supplied oil.
Japanese Patent Laid-Open Publication No. Hei 9-210211 discloses a seal ring 5″ having a plurality of oil grooves 24 on the side faces of the seal ring 5″ to be distanced away from each other (the grooves 24 are opened toward the inner circumferential face of the seal ring 5″), and an inclined concave portion 24′ as a wedge-effect generation face that extends at the circumferential direction side of that oil groove and is connected to that oil groove.
In each of the above two seal rings 5′ and 5″, a pushing force against the wall face of the ring groove is reduced by the pressure from oil introduced into the radially directed grooves 21, 24, the circumferentially directed groove 22, and the concave portion. Therefore the contact pressure between the seal ring and the wall face of the ring groove is reduced. Thus, the above seal rings 5′ and 5″ are effective at reducing friction and improving lubricating function.
However, due to variation in the processing accuracy of the wall face of the ring groove, the ring groove often broadens outward, i.e., the wall face 7′ of the ring groove is often tapered in such a manner that the width of the ring groove at the bottom of the ring groove is smaller than that at the opening thereof, as shown in FIG. 6. When the seal ring shown in FIG. 4 or 6 is used with such a ring groove, an inner circumferential corner of the side face of the seal ring may come into contact with the wall face 7′ of the ring groove. This allows oil that entered the region on the inner circumferential side of the seal ring to leak through gaps with the seal ring away from each other, resulting in degradation of the seal properties of the side face of the seal ring.
A seal ring for overcoming the above problem is described in Japanese Patent Laid-Open Publications Nos. Hei 8-219292 and Hei 9-217836. As shown in FIG. 7, in this seal ring 5′″ a side face 57 is tapered at an angle of 2° to 10° in such a manner that the width of the seal ring on the inner circumferential side is smaller than that on the outer circumferential side. For this seal ring 5′″, since the side face 57 thereof is tapered, contact between the side face 57 and the wall face 7′ of the ring groove does not disengage even if the ring groove broadens outward because of variations in the processing accuracy of the wall face 7′. Thus, this seal ring 5′″ has an advantage in that the sealing properties are not significantly degraded.
Moreover, since the side face of the seal ring is a tapered face that is inclined inward as it travels radially inward, a wedge-like gap 56 that is opened inward in the radial direction is formed between the side face of the seal ring and the sidewall face of the ring groove. Thus, any contact pressure between the seal ring 5′″ and the sidewall face of the ring groove is reduced by the pressure from the oil introduced into that gap 56. Friction is thus reduced.
However, in a case where the groove broadens outward because of variations in the processing accuracy of the wall face of the ring groove, the above-described wedge-like gap is not formed between the side face of the seal ring and the side wall face of the ring groove. Thus, the above-described action of the oil pressure is inadequate and the desired reduction of friction cannot be achieved.
FIG. 8 shows a seal ring 5″″ obtained by improving the above seal ring described in Japanese Patent Laid-Open Publication No. Hei 8-219292. The seal ring 5″″ is provided with a tapered face in such a manner that the width between both side faces of the seal ring on the inner circumferential side is smaller than that on the outer circumferential side.
A feature of this seal ring 5″″ is as follows. Both the side faces are formed to be two-tier tapered faces each of which includes a first inclined face 58 and a second inclined face 59 arranged on the inner circumferential side of the first inclined face 58. Here, the inclination angle of the second inclined face 59 is larger than that of the first inclined face 58. In this example, the inclination angle of the first inclined face 58 is set in the range from 0.5° to 3°, while the inclination angle of the second inclined face 59 is set in the range from 9° to 11°.
This seal ring 5″″ has an advantage that, even if the wall face of the ring groove is tapered like the wall face 7′ where the ring groove broadens outward, contact between the side face of the seal ring 5″″ and the sidewall face of the ring groove does not disengage. This is due to the inclination angle of the first inclined face 58 positioned on the outer circumferential side. Therefore significant degradation of the sealing properties does not occur. Moreover, due to the inclination angle of the second inclined face 59 positioned on the inner circumferential side, the contact pressure between the seal ring 5″″ and the sidewall face of the ring groove is reduced by the action of pressure from oil introduced into the gap 56. Friction is thus reduced.
However, in the seal ring having the one-tier tapered structure, an oil leakage path which releases inside oil pressure to the outside through the wedge-like gap is formed between a top end of a projection having an abutting end structure and an end face of the wall of the ring groove that is opposed to the top end of the projection. Thus, the sealing effect of this type of special abutting end structure cannot be sufficiently maintained.
In addition, the principle of the seal ring having a two-tier tapered structure is the same as the above seal ring of a one-tier tapered structure. Thus, although the seal ring of the two-tier tapered structure can improve sealing properties, there is still a problem with the sealing properties that needs to be overcome.